1. Field of the Invention
The present invention relates to a backside-illumination-type solid-state imaging device in which a light-receiving surface of a photoelectric conversion section is disposed on a backside of a substrate. In particular, the invention relates to a solid-state imaging device that is suitable for use in CMOS image sensors manufactured using a MOS process.
2. Description of the Related Art
Recently, in solid-state imaging devices, such as CCD image sensors and CMOS image sensors, with the reduction in the size of devices to be mounted, the chip area has been reduced and reduction in the area per pixel has been necessitated. As a result, in a solid-state imaging device which receives light from the front side of a chip substrate, where electrodes and lines are placed, light is blocked by the electrodes and lines, and thus it is not possible to have satisfactory light collection characteristics. As a technique to overcome this problem, a backside-illumination-type solid-state imaging device is fabricated in which light is received from the backside of a substrate, where no lines or electrodes are placed, and photoelectric conversion is performed in the substrate to improve light collection characteristics. For example, refer to Japanese Unexamined Patent Application Publication No. 2003-338615 (Patent Document 1).
FIG. 1 is a cross-sectional view showing an example of a typical structure of a backside-illumination-type solid-state imaging device (CMOS image sensor) in the past.
In the solid-state imaging device, light-receiving portions 12 of photoelectric conversion sections (photodiodes) and element isolation regions 14 are disposed in a semiconductor substrate 10, and gate oxide films 16 of MOS transistors, gate electrodes 18, contacts (not shown), wiring films 22 and 26, interlayer insulating films 24 and 28, an insulation covering film 30, etc. are disposed on and above the semiconductor substrate 10.
In such a backside-illumination-type device, light is received from the backside, and light incident on the light-receiving portion 12 in the semiconductor substrate 10 is subjected to photoelectric conversion. Light received from the backside is not entirely subjected to photoelectric conversion in the substrate 10, and as indicated by an arrow A, a part of incident light reaches the front surface of the substrate 10 and is further transmitted through the upper layers above the front surface of the substrate 10. The light transmitted through the upper layers is reflected by the wiring film 26 composed of a metal located in the upper layer above the substrate 10. As indicated by an arrow B, some part of the reflected light enters the photoelectric conversion section again.
When such reflected light returns to and enters the photoelectric conversion section through which the light has been originally transmitted, substantially no adverse effect is caused. However, when the reflected light enters another photoelectric conversion section other than the original photoelectric conversion section, color separation characteristics may be degraded, and flare or the like may occur.
Under these circumstances, a technique has been proposed in which a gate electrode or a gate oxide film is allowed to function as a metal reflection film or a dielectric reflection film to prevent the adverse effect of reflected light.